Optical waveguide fiber with titania-silica outer cladding and method of manufacturing

ABSTRACT

A method of making an optical waveguide fiber with a fatigue resistant TiO 2  -SiO 2  outer cladding, and a substantially glass blank for drawing into such fiber, wherein a glass soot TiO 2  -SiO 2  outermost layer, with an initial TiO 2  concentration greater than 10.5 wt. %, is deposited on a preform, and the preform is exposed to an atmosphere of chlorine and oxygen at a high temperature, and the resulting TiO 2  concentration in the outermost layer of the TiO 2  -SiO 2  outer cladding of the substantially glass blank is less than the initial TiO 2  concentration. In the glass blank form, the outermost layer includes a substantial volume percentage of crystalline phases and in the fiber form, the outermost layer includes inhomogeneities.

This is a continuation-in-part of our prior application Ser. No.456,141, filed Dec. 22, 1989, now U.S. Pat. No. 5,067,975.

BACKGROUND OF THE INVENTION

This invention relates to a method for making an optical waveguide fiberwith a fatigue resistant TiO₂ -SiO₂ outer cladding.

Although glass is a brittle material, the intrinsic strength of pristineglass optical fibers is very high, on the order of 1,000,000 psi forSiO₂ based fibers. Typically, glass optical fibers fail from surfaceimperfections when placed under sufficient tensile stress. Accordingly,much effort has been devoted to the elimination of surface flaws bycareful handling during and after glass forming, by a protective plasticcoating, and by various treatments to the glass surface. In the lattercase, one method of reducing failure by surface flaws is to provide acompressive stress on the glass surface that counteracts applied tensilestresses.

It is well known that flaws in glass grow subcritically prior to failurewhen subjected to tensile stress in the presence of water, ammonia, orother corrosive agents. This phenomenon of subcritical crack growth inglass is known as fatigue and greatly impacts the long-term reliabilityof glass based materials such as glass optical fibers. Therefore, thefatigue performance of optical fiber is especially important to thedesign of low cost fiber cables which have fewer strength members andless environmental protection than standard optical telecommunicationscables.

It has been known for some time that the strength of a glass body may beincreased by forming its surface region from a glass with a thermalcoefficient of expansion that is lower than the thermal coefficient ofexpansion of the interior glass. As the combination is cooled from hightemperatures, this configuration places the glass surface incompression, thereby inhibiting the formation and growth of cracks. See,e.g.: Giffen et al. U.S. Pat. No. 3,673,049; and Krohn and Cooper,"Strengthening of Glass Fibers: I, Cladding", Journal of the AmericanCeramic Society, Vol. 52, No. 12, pp. 661-4, Dec. 1969.

Numerous attempts have been made to create a strengthened optical fiberwith such a compressive surface layer. See, Maurer et al. U.S. Pat. No.3,884,550; MacChesney et al., "Low Loss Silica Core-Borosilicate CladFiber Optical Waveguide", American Ceramic Society Bulletin, Vol. 52, p.713, 1973. Macedo U.S. Pat. No. 4,181,403 refers to compression in athin surface layer formed by "molecular stuffing" in fiber with a largeoptical core and very thin optical cladding. Some of these attemptsinvolved the use of a TiO₂ -SiO₂ outer layer on the fiber, as itsthermal coefficient of expansion is known to be less than that of SiO₂.See, e.g.: Schneider et al. U.S. Pat. No. 4,184,860; Kao et al. U.S.Pat. No. 4,243,298; and, Taka et al. Japanese Patent No. 1,255,795.

Schneider et al. U.S. Pat. No. 4,184,860 describes an outer TiO₂ -SiO₂layer with 8 wt. % TiO₂ surrounding a 15 wt. % TiO₂ layer which is heattreated (by "tempering") to devitrify and partially separate and/orcrystallize. This heat treatment of the 15 wt. % TiO₂ intermediate layeris intended to raise the thermal coefficient of expansion so that it issubstantially greater than the coefficient of the outer TiO₂ -SiO₂layer, thereby putting the outer layer in compression. Thus, theSchneider et al. fiber design relies on the 8 wt. % TiO₂ outer layer toprovide enhanced strength through compression.

Schultz studied SiO₂ -TiO₂ glasses containing 10-20 wt. % TiO₂ whichwere clear when formed, but which exhibited increased opacity from phaseseparation and anatase formation, along with large changes in thermalexpansion, upon heat treatment at temperatures below the annealingpoint. "Binary Titania-Silica Glasses Containing 10 to 20 Wt. % TiO₂ ",Journal of the American Ceramic Society, Vol. 58, No. 5-6, May-June 1976(Schultz U.S. Pat. No. 3,690,855). By studying the physical propertiesof these TiO₂ -SiO₂ compositions, Schultz described three glass formingregions as stable (0-10 wt. %), metastable (10-18 wt. %) and unstable(>18 wt. %).

Some recent research has been directed toward understanding themechanism of crack growth in SiO₂ glass on the molecular level. See,Michalske and Bunker, "The Fracturing of Glass", Scientific American,Dec. 1987, pp. 122-129. The Michalske and Bunker paper presents anatomistic study of glass fracture in the presence of water, but islimited to homogeneous SiO₂ glass. Additional research has been directedtoward crack growth in continuous fiber filled composites. See,Michalske and Hellmann, "Strength and Toughness of Continuous-AluminaFiber-Reinforced Glass-Matrix Composites," Journal of the AmericanCeramic Society, Vol. 71, No. 9, pp. 725-31, Sep. 1988.

SUMMARY OF THE INVENTION

Our invention resides in a manufacturing process for producing amarkedly superior fiber design which provides a surprising improvementin fatigue resistance.

In accordance with one aspect of our invention, a manufacturing processfor a glass blank to be drawn into an optical waveguide fiber isprovided, including depositing glass soot in the form of a soot preformincluding an outer cladding of TiO₂ -SiO₂ with an outermost layer havingan initial TiO₂ concentration greater than 10.5 wt. %, exposing thepreform to an atmosphere containing chlorine at a temperature in therange of about 900° C. to about 1400° C., for a time sufficient todehydrate and consolidate the preform into the substantially glassblank, wherein the resulting TiO₂ concentration in the outermost layerof the TiO₂ -SiO₂ outer cladding of the substantially glass blank isless than the initial TiO₂ concentration.

In accordance with another aspect of our invention, a method of making afatigue resistant optical waveguide fiber with a TiO₂ -SiO₂ outercladding, is provided including forming a doped SiO₂ preform with a coreportion and a cladding portion, depositing a layer of TiO₂ -SiO₂ soot onthe outside of the cladding portion to create an augmented preform, theTiO₂ -SiO₂ layer including at least one sub-layer having a TiO₂concentration greater than 10.5 wt. %, exposing the augmented preform toan atmosphere containing chlorine at a temperature within the range ofabout 900° to 1400° C., consolidating the preform into a substantiallyglass blank, the exposing and consolidating steps resulting in greaterthan about 2 volume percent TiO₂ crystalline phases with diametersgreater than or equal to about 0.3 μm within the TiO₂ -SiO₂ layer of thesubstantially glass blank, and drawing the substantially glass blankinto an optical waveguide fiber with inhomogeneities in the outer TiO₂-SiO₂ layer of the fiber.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of dynamic fatigue (n value) vs. TiO₂ concentration.

FIGS. 2a-2d are photomicrographs of the outer TiO₂ -SiO₂ layers ofoptical fibers, using STEM techniques.

FIG. 3 is a rough flow chart of a manufacturing process for makingoptical fiber with a TiO₂ -SiO₂ outer layer.

FIGS. 4a-4b are TEM photomicrographs of agglomerations of TiO₂ -SiO₂soot particles.

FIGS. 5a-5c are computer simulated maps of SEM photomicrographs of partof the outer TiO₂ -SiO₂ layer of three different consolidated glassblanks.

FIGS. 6a-6c are the SEM photomicrographs related to the computersimulated maps in FIGS. 5a-5c.

FIGS. 7a-1, 7a-2, 7b-1, 7b-2, 7c-1 and 7c-2 are graphs of TiO₂concentration vs. layer thickness for the glass blanks depicted in FIGS.5a-5c and 6a-6c, as measured with electron microprobe techniques.

FIG. 8 is a graph showing ease of manufacturing for optical fibershaving TiO₂ -SiO₂ outer claddings as a function of layer thickness andTiO₂ concentration.

FIGS. 9a-9b are graphs of TiO₂ concentration vs. radial position in anouter TiO₂ -SiO₂ cladding layer of a consolidated glass blank, asmeasured with SEM techniques.

FIG. 10 is a graph of TiO₂ concentration vs. radial position in theouter TiO₂ -SiO₂ cladding layer of the optical fiber depicted in FIGS.9a and 9b, as measured with STEM techniques.

FIG. 11 is a graph of intrinsic optical fiber strength vs. TiO₂concentration for a large number of optical fiber samples.

FIG. 12 is a drawing of a burner endface for use in flamehydrolysis/oxidation deposition.

DETAILED DESCRIPTION

We have found that inhomogeneities in TiO₂ -SiO₂ outer cladding layersprovide new mechanisms for crack growth resistance in optical waveguidefibers Some of the possible explanations of this property are: 1)alteration of the SiO₂ network by the presence of TiO₂, so that when thenetwork is stressed, it has more options for accommodating the appliedstress or greater compliance (this correlates with a substantialdecrease in Young's modulus); 2) the tendency of high TiO₂ concentrationinhomogeneities to expand upon cooling, placing a residual stress on acrack in their vicinity and acting as a means of crack closure; 3) cracktip deflection by the inhomogeneities (the stress intensity at the cracktip is greatly decreased when the crack is directed away from thedirection normal to the applied tensile stress); 4) the resistance ofthe inhomogeneities to transgranular fracture; 5) microcrack tougheningwhere a crack encountering an inhomogeneity initiates several smallercracks out of the matrix/inhomogeneity interface (the creation ofmultiple cracks from a single crack is a source of strain energydissipation); and, 6) crack growth resistance via crack tip shielding byinhomogenities. Some of these fatigue resistance mechanisms havesimilarities to mechanisms found to be active in glass-ceramics and infiber and whisker reinforced composites. See, Michalske and Hellmann,"Strength and Toughness of Continuous-Alumina Fiber-ReinforcedGlass-Matrix Composites," Journal of the American Ceramic Society, Vol.71, No. 9, pp. 725-31, Sep.1988.

It is well recognized that the current understanding of how flaws growsubcritically in glass fibers is in its infancy. The complicatingfactors are, inter alia, that crack growth cannot be directly observeddue to extremely small flaws, that strength and fatigue measurementsthat are statistical in nature must be used to infer crack growth, andfurthermore, that flaws remaining after proof stress are so infrequentthat their fatigue behavior must be simulated by artificially introduceddefects added during fatigue testing. These difficulties requirecomplicated testing with results that are oftentimes counter-intuitive.Thus, test results and theories must receive careful study.

In this patent application, it is assumed that fracture mechanicsapplies to flaws in glass optical fibers: namely, that the stressintensity factor, K_(I), is related to the applied tensile stress,σ_(a), and flaw depth, a, by

    K.sub.I =0.73σ.sub.a (πa).sup.1/2                 (1)

When K_(I) reaches the fracture toughness, K_(I) =K_(Ic), failure occursand the above equation can be rearranged to give strength, σ_(f), as afunction of crack depth, a,

    σ.sub.f =K.sub.Ic /0.73(πa).sup.1/2               (2)

It is also assumed that the power law crack velocity model describes therelationship between crack velocity and stress intensity factor by,

    V=AK.sub.I n                                               (3)

where A and n are crack growth parameters. The crack growth parameter nis of particular value in that it gives a measure of a material'ssusceptibility to subcritical crack growth. For optical fibers n isoften measured using the dynamic fatigue technique where fiber strength,σ_(f), is measured as a function of stress rate, σ_(r), where, ##EQU1##The subscripts 1 and 2 indicate different measured strengths fordifferent rates of stress. The value for n is determined by simpleregression of log strength versus log stress rate where the slope isequal to 1/(n+1). For a general discussion of the measurement of fatigueresistance n value, see Glaesemann, Jakus, and Ritter, "StrengthVariability of Indented Soda-Lime Glass", Journal of the AmericanCeramic Society, Vol. 70, No. 6, Jun. 1987, pp. 441-444.

For the n values given herein, fiber strength was measured in 100%relative humidity at approximately 25° C. using 20 meter gauge lengths.The stress rates used correspond to strain rates of 4 and 0.004%/min.The standard deviation of the slope {1/(n+1)} was typically 10% of themean for the n values reported herein. A similar, but not as exhaustive,dynamic fatigue test technique is given in E.I.A. test procedureFOTP-76. Strength data set forth herein (as opposed to fatigueresistance data) were measured with the 4%/min. strain rate under theabove environmental conditions.

FIG. 1 depicts the measured n values graphed vs. TiO₂ concentration fora series of fibers with varying TiO₂ concentration, TiO₂ -SiO₂ layerthickness, and manufacturing conditions (note: the concentrationsplotted by the connected open squares depict both concentrations of atwo layer outer cladding--see the discussion below re.: FIG. 2a for adetailed description). The following information is given for each fibertype in the graph: layer thickness; whether a higher concentrationoutermost layer was included (the use of such layers is discussedbelow); and, the gases used in dehydration/consolidation (also discussedbelow).

FIG. 1 depicts the surprising increase in fatigue resistance in ourinventive fibers. In the discussion below, we shall describe theinhomogeneous TiO₂ -SiO₂ outer cladding structures in our inventivefibers which may explain this surprising increase in fatigue resistance.As can be seen from FIG. 1, in optical waveguide fibers with a thinouter cladding of TiO₂ -SiO₂, fatigue resistance increases withincreasing TiO₂ concentration. Up to 10-11 wt. % TiO₂, fatigueresistance increases smoothly with increasing TiO₂ concentration. Aboveabout 10-11 wt. % TiO2, we have found an unexpected and dramaticincrease in n values. The trend appears to begin at about 10-11 wt. %,as indicated by the last few closed circles in FIG. 1, with n valuesfrom about 30-37.

As seen from FIG. 1, above about 10-11 wt. % TiO₂, n values increaseabove the level that would be expected from a linear extrapolation ofthe n values for lower TiO₂ concentrations. Such a linear extrapolationis given by the equation, n=1.29 W+19.77, where W is the weight percentTiO₂. For example, the predicted n value for 15 wt. % would be 39.1, thepredicted n value for 17 wt. % would be 41.7, and the predicted n valuefor 20 wt. % would be 45.6.

As shown in FIG. 1, the n value for the fiber designated with an opencircle around 13.4 wt. % TiO₂ is about 55, much greater than the n valueof about 30 measured for the fiber designated by a closed circle around10 wt. % TiO₂. The fiber with the 13.4 wt. % TiO₂ layer is depicted inFIG. 2c, and FIG. 2c shows that the fiber included a substantial volumeof inhomogeneities. A similar fiber with a 2.5 μm TiO₂ -SiO₂ layer ofabout 12.6 wt. % TiO₂ had an n value of about 54; this n value isdesignated by the other open circle in FIG. 1. The fibers designated bythe open squares and open diamonds show an even more dramatic increase,up to an n value of 87. The apparent anomaly in the results associatedwith the fibers designated by open inverted triangles is discussed belowwith respect to the manufacturing process used to make these fibers.

The data indicate that the highly superior properties of TiO₂ -SiO₂ cladfiber are not the result of bulk compressive stresses on the claddingsurface as believed in the prior art, but are due to the inhomogeneousstructure of the material. It is important to note that since n is usedas an exponent, for the increases in n value depicted in FIG. 1 withrespect to higher TiO₂ concentrations, the increase in fatigueresistance in our inventive fibers is even more dramatic than the graphimplies.

A. STRUCTURE OF TIO₂ -SIO₂ LAYER

It is instructive to consider TiO₂ -SiO₂ compositions in four forms: 1)bulk glass in general; 2) low density soot preforms; 3) higher densityglass blanks after dehydration/consolidation; and, 4) optical fibersafter drawing. For TiO₂ -SiO₂ compositions in general, at TiO₂concentrations below the eutectic (about 10.5 wt. % TiO₂), the TiO₂appears to be both randomly dispersed in the SiO₂ matrix and present inclusters of 5- and 6- fold coordinated Ti ions (4- fold coordinated Tiions are less likely to occur in clusters and are only clustered incombination with 5- or 6- fold coordinated Ti ions). More specificcharacterizations of such compositions in drawn fiber are providedbelow. As TiO₂ concentrations increase above the eutectic, theseclusters are nucleating sites for somewhat spherical phase separatedregions or "inhomogeneities" which grow in size and may begin todevitrify as crystalline TiO₂.

In the region of maximum glass stability--below the eutectic--theseinhomogeneities have not been observed: i.e., there are no significantlevels of clusters greater than about 10 Angstroms in diameter, theresolution limit for our conventional Scanning Transmission ElectronMicroscope (STEM) instrument (note: other instruments or techniques maybe capable of resolving phase separated TiO₂ -SiO₂ at even smallerdiameters). Based on molar volume calculations, we believe that aninhomogeneity detectable by STEM techniques would contain at least about80 Ti atoms.

It is also possible to detect phase separation (i.e., to distinguish aninhomogeneity from a cluster) when the inhomogeneity reaches asufficient size that discontinuities appear in macroscopic physicalproperties such as thermal expansion coefficient, density, refractiveindex, volume of mixing, strain and anneal points as a function ofconcentration. Such discontinuities have been correlated classically toa transition from an amorphous to a phase separated state.

We have found that after drawing at temperatures above about 1900° C.,optical fiber TiO₂ -SiO₂ layers are amorphous glass at TiO₂concentrations below about 11 wt. %. However, we believe that the Tiions are not randomly dispersed in the SiO₂ matrix but tend to clusterwith other Ti ions in 5- and 6-fold coordination, the same as in bulkglasses as described above. Valence bond theory suggests that it isunlikely that an isolated 5- or 6- fold coordinated Ti ion exists in theglass network: each such Ti ion would likely be linked to at least oneother Ti ion, resulting in clustering. It should also be noted that thecoordination of the Ti ions in optical fiber may be affected by the fastquench associated with the fiber drawing process and the presence ofcrystalline TiO₂ in the glass blank from which the fiber is drawn.

Clusters in compositions below about 11 wt. % TiO₂ are smaller than thecritical size required for nucleation either as a separate liquid orcrystalline phase in the draw process. Below this TiO₂ concentration,microscopic properties of the glass network control the mechanicalperformance of the resulting fibers, while above this level,"macroscopic" effects due to phase separation and crystallizationdetermine the fibers' mechanical performance. In addition, as explainedbelow, dehydration/consolidation in Cl₂ increases the crystallization inthe glass blank from which fibers are drawn, and such fibers appear tohave a greater degree of phase separation.

For compositions below about 11 wt. % TiO₂, and in the homogeneous glassmatrix for higher TiO₂ compositions, we believe that the enhancedfatigue resistance of TiO₂ -SiO₂ over SiO₂ may be explained by referenceto bond force constants rather than bond energy, and by reference to thestrong likelihood of clustered Ti ions in the glass.

Considering bond energy alone, TiO₂ -SiO₂ compositions would appear tobe weaker than pure SiO₂, as the Ti-O bond strength as reported in theliterature is on the order of 70 Kcal/mole as opposed to 110 Kcal/molefor the Si-O bond. However, consideration of bond force constants leadsto a very different result. In TiO₂ -SiO₂ compositions, regardless ofthe Ti ion coordination, there are numerous Ti-O-Si linkages. Comparedto an Si-O-Si linkage, the oxygen atom in a Ti-O-Si linkage resides in amore asymmetric, broadened potential well. Such broadening makes manymore vibrational states accessible to the system, in effect making thesystem as a whole "softer". Because of the looseness of this arrayrelative to the more rigid Si-O-Si environment, the Ti-O-Si linkagesformed as a result of TiO₂ addition will function as high energydissipating regions to remove energy at the stress point of the cracktip. Clustering of 5- and 6- fold coordinated Ti ions would result ineven greater enhancement of fatigue resistance and extended regions ofenhanced energy dissipation in the glass, as the potential wells for^(IV) Si-O-^(V) Ti and ^(IV) Si-O-^(VI) Ti linkages would be even moreasymmetric than for ^(IV) Si-O-^(IV) Ti, and therefore the system wouldbe even "softer".

At compositions above around 11 wt. % TiO₂ where phase separated TiO₂-SiO₂ becomes visible, the Ti-O-Si linkages within each phase domain andthe Ti-O-Si linkages forming the interface between the TiO₂ -richinhomogeneity and the SiO₂ -rich matrix are very important to enhancedfatigue resistance. Our analysis indicates that for TiO₂ -SiO₂ bulkglass in general at TiO₂ concentrations above the eutectic, thecomposition of the phase separated domains or inhomogeneities isapproximately the same, viz. 92-95 wt. % TiO₂ in at least an 11 wt. %TiO₂ matrix. It is conceivable that the matrix concentration could be ashigh as 19 wt. % TiO₂.

For all TiO₂ concentrations we have studied below about 19 wt. %, webelieve the number of inhomogeneities increases with increasing overallTiO₂ concentration, but the size and composition remain approximatelythe same. Similarly, the concentration of TiO₂ dissolved as clustered Tibelow the inhomogeneity size threshold in the SiO₂ -rich matrixstabilizes at a maximum near the eutectic bulk composition at 11-13 wt.% TiO₂.

The structure and composition of the soot as laid down, and of the glassblank after dehydration/consolidation are discussed below in connectionwith a description of the manufacturing process. In the drawn fiber, forthe concentration regions studied, the proportion of inhomogeneities mayincrease to more than 50 vol. %. During the draw process, the large TiO₂crystals (anatase, and rutile at the higher concentrations) in the glassblank dissolve into a TiO₂ -SiO₂ melt at temperatures above about 1900°C., and subsequently precipitate out as much smaller phase separateddomains or inhomogeneities in the quenched fiber, as the fiber rapidlycools to below about 1550° C.

In the drawn fiber, a substantial portion of the inhomogeneities we haveobserved are between 10 and 100 Angstroms in diameter, typicallyapproximately 30-50 Angstroms. For inhomogeneities of this scale, thecomposition of each phase region cannot be measured even by electronmicrosopy techniques. In the drawn fiber, most of the inhomogeneities wehave observed appear to be phase separated regions without substantialcrystal content, although we have observed a few inhomogeneities whichappear darker in STEM photomicrographs, suggesting crystal content. Forthe purposes of this application, detectable crystalline content in afiber layer shall mean a substantial number of crystals with diametersgreater than about 200 Angstroms.

The number and volume percentage of phase separated and crystalline TiO₂domains increase with increasing TiO₂ concentration. As described below,drying the preform in Cl₂ will increase the likelihood of phaseseparation and potential crystallization in the fiber. In regions offibers with TiO₂ concentration near the eutectic, if phase separationdoes occur, the domains reflect volume percentage and distribution thatis similar to the levels of anatase crystals within the blank. The fibermay show discrete regions of phase separated TiO₂ -SiO₂ (where anatasehad dissolved into the glass during draw and subsequently precipitatedout) in a TiO₂ -SiO₂ matrix glass that is itself not phase separated atthat TiO₂ concentration. In effect, the development of large anatase(and perhaps rutile) crystals in the blank in thedehydration/consolidation process results in the onset of liquidimmiscibility in the fiber at TiO₂ concentrations lower than thoseexpected from equilibrium considerations and from previousinvestigations of TiO₂ -SiO₂ bulk glass compositions in general. Atconcentrations exceeding about 13 wt. % TiO₂, much more extensive,continuous and uniform phase separation is apparent in the fiber.

STEM photomicrographs of the inhomogeneities as present in opticalfibers of varying concentration, layer thickness and manufacturingconditions are shown in FIGS. 2a-2d.

The fiber depicted in FIG. 2a had a 3.5 μm TiO₂ -SiO₂ outer layerincluding a 3.1 μm first layer with 14.7 wt. % TiO₂ (end-on SEMmeasurement of the fiber and electron microprobe of the blank), and anhigher concentration layer with about 16.7-17 wt. % TiO₂ (16.7 wt. %:electron microprobe of the blank; 17 wt. %: as extrapolated from thedeposition flows). The soot preform was dehydrated/consolidated in Cl₂with a small amount of O₂ introduced by a leaking valve. The measured nvalue was 87. The region of the fiber surface is indicated by "a" andthe region of inhomogeneities is indicated by "b". The n valuemeasurements for this fiber are designated by the open squares in FIG.1, and the intrinsic strength measurements are designated by the opensquares in FIG. 11. FIGS. 5a, 6a, 7a-1 and 7a-2 also relate to thisfiber. The process for manufacturing this fiber is described below atthe end of Example 1.

The fiber depicted in FIG. 2b had a 1.1 μm TiO₂ -SiO₂ layer (roughlyuniform TiO₂ concentration) that was dehydrated/consolidated in Cl₂without O₂. The TiO₂ concentration as extrapolated from the depositionflows was 17.4 wt. % and the measured n values for this fiber were 77.8and 80.3. The region of inhomogeneities is indicated by "b". The n valuemeasurements for this fiber are designated by the open diamonds in FIG.1, and the intrinsic strength measurements are designated by the opendiamonds in FIG. 11. The process for manufacturing this fiber isdescribed below in Example 3. The precise TiO₂ concentration of thisfiber would be difficult to measure by SEM techniques, as the typicalSEM beam spot depth is greater than 1 μm; an SEM measurement wouldalways give a minimum concentration for at least one layer in the fiber,as the measured value would be reduced by the SiO₂ interrogated by thedeeper portion of the SEM beam.

The fiber depicted in FIG. 2c had a 2.5 μm TiO₂ -SiO₂ layer (roughlyuniform TiO₂ concentration) that was dehydrated/consolidated in Cl₂ andO₂. The end-on SEM measurement of TiO₂ concentration in the fiber was13.4 wt. % and the measured n value was 54.6. The region ofinhomogeneities is indicated by "b". The n value measurement for thisfiber is designated by the higher TiO₂ concentration open circle in FIG.1.

The fiber depicted in FIG. 2d had a 3.5 μm TiO₂ -SiO₂ outer layerincluding a 3.1 μm first layer with 10.9 wt. % TiO₂ (end-on SEMmeasurement of the fiber), and a 0.4 μm outermost higher concentrationlayer with about 16.0 wt. % TiO₂ (as extrapolated from the depositionflows). The TiO₂ -SiO₂ layer was dehydrated/consolidated in Cl₂ and O₂and the measured n value was 41.3. The fiber surface is indicated by "a"and the region of inhomogeneities is indicated by "b". The n valuemeasured for this fiber indicates that a high TiO₂ concentration in thefirst primary layer of such a two layer fiber would be preferable forachieving extremely high n values.

B. MANUFACTURING PROCESS

As described above, one aspect of the present invention relates toprocesses for manufacturing optical fibers with fatigue resistant TiO₂-SiO₂ outer claddings. The methods of the invention are particularlysuitable for use with the outside vapor deposition (OVD) and the vaporaxial deposition (VAD) soot laydown processes. OVD processes aredescribed in Berkey U.S. Pat. No. 4,453,961 and further described inBerkey U.S. Pat. No. 4,486,212, and in the various patents referred toin those patents, the pertinent portions of all of which areincorporated herein by reference. VAD processes are described in OpticalFiber Communications, vol. 1, 1985, Bell Telephone Laboratories, Inc.section 3.3, pp. 100-116, and in U.S. Pat. No. 4,367,085.

A flow diagram of one method in accordance with the present invention isset forth in FIG. 3. In this process, an additional laydown of one ormore TiO₂ -SiO₂ soot layers is provided at the end of a conventional OVDsoot laydown process. In one embodiment, this additional laydown step isincluded in the process for manufacturing a single unitary soot preformwith a core region and a cladding region, as described in Berkey U.S.Pat. No. 4,486,212. In another embodiment, the additional laydown stepis provided at the end of an overcladding process such as is alsodescribed in Berkey U.S. Pat. No. 4,486,212 whereby a large diameterintermediate fiber comprising the core region and a portion of thecladding region is overcoated with additional cladding soot. It is knownin the art that OVD and VAD soot laydown may be carried out with aplurality of burners, as described in Berkey U.S. Pat. No. 4,684,384 andPowers U.S. Pat. Nos. 4,378,985 and 4,568,370.

The additional laydown of a TiO₂ -SiO₂ layer is carried out as follows.The SiCl₄ vapor is provided to the burner by a reactant delivery systemof the type described in Blankenship U.S. Pat. No. 4,314,837. Inaddition, the TiCl₄ vapor is provided to the burner by a flashvaporization system as described in copending Antos et al. U.S. patentapplication Ser. No. 07/456,118, entitled Flash Vaporizer System for Usein Manufacturing Optical Waveguide Fiber, and filed concurrentlyherewith, which is incorporated herein by reference.

SiO₂ soot consists of agglomerations of glass soot particles withdiameter in the range from about 0.1 to 0.3 μm. It is believed that TiO₂-SiO₂ soot exists in three separate forms: a) agglomerations ofparticles of roughly homogeneous solutions of TiO₂ in SiO₂, with aboutthe same diameter as SiO₂ soot particles; b) tiny anatase crystallinefines on the surface of these particles, typically less than about 90Angstroms in diameter (these fines being more prevalent in compositionswith greater than about 10.5 wt. % TiO₂); and, c) larger anatasecrystals agglomerated with the particles, typically between 200 and1,000 Angstroms in diameter. These three forms are shown in FIGS. 4a and4b (TEM photomicrographs) as "a", "b" and "c". This soot was measured bywet chemical analysis, and by extrapolation from the deposition flows,to be 13 wt. % TiO₂.

X-ray diffraction (XRD) can be used to roughly quantify the volumepercentage of TiO₂ crystals above around 200 Angstroms in diameter atlevels above about 0.1 vol. %. Transmission Electron Microscopy (TEM)may be used to detect the crystalline fines, but it is notsatisfactorily quantitative. Although the presence of anatase in thesoot was confirmed by TEM, XRD is unable to quantify large anatasecrystals in the soot until the TiO₂ concentration exceeds about 9 wt. %.In the soots we studied, concentrations of up to about 1 vol. % crystalswere found in soot with TiO₂ concentration of up to about 13 wt. %.

It is theorized that the TiCl₄ and SiCl₄ react at approximately the sametemperature in the flame, forming the roughly homogeneous glassparticles, except where the TiCl₄ can react with H₂ O at temperaturesless than approximately 1600° C. As the solubility limit of TiO₂ in SiO₂is exceeded, the fines of TiO₂ may be precipitated from the moltenparticles. The larger anatase crystals may be formed by the reaction ofTiCl₄ with H₂ O at temperatures less than about 1600° C. in the coolercenterline of the burner flame. In one embodiment of the invention, thetwo forms of anatase are uniformly distributed throughout the TiO₂ -SiO₂layer as a function of layer concentration. The size and prevalence ofanatase crystals in the soot preform may be increased by the presence oraddition of H₂ O in the deposition flame reaction.

After laydown, the soot preforms are dehydrated and consolidated,typically in a chlorine atmosphere, as described in DeLuca U.S. Pat. No.3,933,454, Powers U.S. Pat. No. 4,125,388, and Lane et al. U.S. Pat. No.4,741,748. Pertinent portions of these patents are also incorporatedherein by reference. The dehydration and consolidation steps can becarried out simultaneously or in two different steps, provided thatrewetting of the dehydrated preform is avoided by the use of a dry inertgas atmosphere or other means. In an alternative embodiment, thedeposition of the TiO₂ -SiO₂ outer cladding layer may be carried outafter the dehydration/consolidation of the rest of the preform, and theresultant preform with a soot outer cladding layer may be thereafterdehydrated, or otherwise treated with chlorine, and consolidated.

If no movement of the TiO₂ occurred in the fiber making processsubsequent to laydown (i.e., in dehydration/consolidation and in draw),it would be preferable for the anatase to be uniformly distributed inthe soot preform in order to achieve uniform distribution of TiO₂ and/orinhomogeneities in the drawn optical fiber. However, we have discoveredthat for higher concentrations of TiO₂ in the soot preform, the use ofchlorine in dehydration/consolidation results in TiO₂ transport, crystalgrowth and surface depletion.

A substantial fraction of anatase crystals between 0.05 and 5 μm,typically around 0.5 to 1.5 μm, are found in the solid glass blank afterdehydration/consolidation. Depending on the TiO₂ concentration anddehydration/consolidation conditions, the concentration of crystallineTiO₂ above 0.3 μm in diameter in the glass blank increased from smallvol. % at 8 wt. % TiO₂ to over 5 vol. % at about 14 wt. %. The largepopulation of crystals between 0.05 μm and 0.3 μm could not bequantified. However, as the size distribution of the crystals mapped bySEM was the largest at the smallest crystal diameters detected, thepopulation of the crystals below 0.3 μm may be at least as large.

It is believed that, at temperatures above about 900° C., the chlorineattacks anatase-rich regions in the soot preform duringdehydration/consolidation but does not attack homogeneous TiO₂ -SiO₂glass regions. In addition, this attacked Ti is transported andredeposited on other anatase crystals, resulting in the elimination ofanatase fines and growth of larger anatase (or rutile) crystals in thefully consolidated glass blank. There is also depletion of the anatasenear the surface of the glass blank.

A significant proportion of the anatase in the preform is grown to sizesabove 0.3 μm, so that in the glass blank, these crystals were observableby Scanning Electron Microscopy (SEM) measurements of crystals >0.3 μm(see the computer simulated maps generated from the SEM data, FIGS.5a-5c, and the direct SEM photomicrographs, FIGS. 6a-6c). The fibersurface in these FIGS. is indicated by "a". FIGS. 7a-1, 7a-2, 7b-1,7b-2, 7c-1 and 7c-2 depict electron microprobe measurements of TiO₂concentrations in consolidated glass blanks. The spikes in FIGS. 7a-2and 7c-2 are due to the presence of large crystals designated by 70. TheSEM measurements for 7a-2 were made in the region of the blank above theroot portion, where the crystals were still apparent to the unassistedeye. The "-2" plots are based on higher resolution measurements of thesurface regions of the outer cladding layers whose measured TiO₂concentrations are depicted in the related "-1" plots. FIGS. 5a, 6a and7a-1 and 7a-2 are associated with fiber "a" which is described abovewith reference to FIG. 2a. In FIGS. 7a-1 and 7a-2, the fiber surface isat the right side of the graph.

FIGS. 5b, 6b and 7b-1 and 7b-2 are associated with fiber "b" which had a3.0 μm TiO₂ layer with a 1.4 μm first layer of approximately 5.5 wt. %TiO₂, a 1.0 μm first higher concentration layer of approximately 8.0 wt.% TiO₂, a 0.35 μm second higher concentration layer of approximately 12wt. % TiO₂, and a 0.25 μm third higher concentration layer ofapproximately 15.5 wt. % TiO₂. These concentrations were extrapolatedfrom electron microprobe measurements of the glass blank; the end-on SEMmeasurement of the fiber was 10.1 wt. %. The n value of fiber "b" wasmeasured to be 46. The blank was dehydrated/consolidated in anatmosphere of Cl₂ without O₂. In FIGS. 7b-1 and 7b-2, the fiber surfaceis at the right side of the graph.

FIGS. 5c, 6c and 7c-1 and 7c-2 are associated with fiber "c" which had a1 μm [Backer: 3 μm?] TiO₂ layer (roughly uniform TiO₂ concentration)with about 13.8 wt. % TiO₂ (measured by electron microprobe on the glassblank). The blank was consolidated in an atmosphere of Cl₂ and O₂. InFIG. 7c-1, the fiber surface is at the left side of the graph, and inFIG. 7c-2, the fiber surface is at the right side of the graph.

The scale of FIGS. 5a-5c is 1"=34.4 μm. The photomicrographs in FIGS.6a-6c are of the outer portion of the TiO₂ -SiO₂ outer cladding layer inthe glass blank. The simulated maps (FIGS. 5a-5c) use a slightlydifferent resolution and depict more of the outer cladding layer. TheSEM photomicrographs in FIGS. 6a-6c were taken with a 25 kvolt 70Angstrom beam that was rastered over the sample to get an image. Theelectron microprobe measurements in FIGS. 7a, 7b-1, 7b-2, 7c-1 and 7c-2were taken as follows with a 15 kvolt beam: 7a-2 μm beam, 50 μm steps;7b-1 - rastered over a grid 50 μm square with 50 μm steps; 7b-2 - 70Angstrom spot with 2.5 μm steps; 7c-1 - 50 μm spot with 50 μm steps;and, 7c-2 - 1 μm spot with 1 μm steps. For the SEM measurements, thespot size is the two dimensional diameter. The beam depth at 15 kvolt isabout 1.5 μm and the beam pattern is pear shaped in the 3rd dimension.

It is believed that the attacked anatase near the lower density surfaceof the preform diffuses quickly to the surface and is transported awayfrom the preform, resulting in depletion of the TiO₂ in the blanksurface layer. In contrast, in the typically higher density interior ofthe preform, the transportable TiO₂ is trapped in the preform and nosignificant Ti loss occurs. Moreover, in glass blanks originally laiddown by the OVD process, there may be gradients of crystal concentrationacross the blank depending on the local density variations created byconsecutive laydown passes. A possible explanation is that "channels"remain between soot pass layers providing axial flow paths for Ti thatis transported during dehydration/consolidation, resulting in increasedlocal redeposition.

The reaction chemistry is believed to be:

    TiO.sub.2 +Cl.sub.2 <=>TiOCl+ClO

    TiOCl+Cl.sub.2 <=>TiOCl.sub.3                              (5)

    TiOCl.sub.3 +ClO<=>TiCl.sub.4 +O.sub.2

As explained below, the presence of O₂ during dehydration/consolidationinhibits migration of Ti. However, as the above equations indicate,although O₂ suppresses TiCl₄ formation, it cannot depress the formationof the various mobile titanium oxychloride species, and therefore cannoteliminate the possibility of TiO₂ depletion. O₂ is effective in reducingTiO₂ depletion because it represses the third reaction above which tendsto be irreversible, thereby forcing the reactions back toward the TiO₂product. The overall reaction mechanism is first order proportional tothe Cl₂ concentration (as experimentally observed). The actual magnitudeof TiO₂ depletion will also be a function of temperature (higher T,faster rate), O₂ concentration (more O₂, less depletion), flow rate(higher flow, greater depletion), and time (the longer the exposure, themore depletion and the greater the likelihood that the preform will beaffected by dynamic flow stripping in the furnace).

The percentage chlorine used in drying impacts the average anatase sizeto a greater extent than does the TiO₂ concentration, with higherpercentage chlorine resulting in larger anatase crystals in the glassblank. Higher percentage chlorine also results in substantiallyincreased surface depletion. Significant (>1 wt. %) depletion at thesurface does not appear to be present for TiO₂ concentrations belowabout 5 wt. %, for preforms dehydrated/consolidated in Cl₂ without O₂,inasmuch as Cl₂ attacks crystals rather than glass and the crystallevels in the soot are minute below this TiO₂ concentration.

We believe that the increase in n values is especially pronounced foroptical fibers whose precursor blanks were dehydrated/consolidated in aCl₂ atmosphere, as depicted by the open circles, open squares and opendiamonds in FIG. 1, as contrasted with the open inverted triangles atabout 12.5 wt. %. However, for high TiO₂ concentrations, acceptably highn values can be achieved even without the use of Cl₂ indehydration/consolidation, as higher volume percentages ofinhomogeneities are present in any event, as the TiO₂ concentration isincreased to higher levels. It should be noted that the open invertedtriangles in FIG. 1 indicate that at TiO₂ concentrations relatively nearthe 11 wt. % discontinuity, Cl₂ may be a significant factor in achievingenhanced n values. The open inverted triangle at about 7 wt. % in FIG. 1indicates that Cl₂ is not likely to be a factor at lower TiO₂concentrations.

In fiber drawn from blanks consolidated without chlorine,inhomogeneities are less apparent. It is believed that no significantgrowth in anatase crystals occurs if Cl₂ is not present duringdehydration/consolidation, and the anatase populations in the glassblank reflect the distribution found in the soot preform - theconcentration of crystals greater than 0.3 μm in diameter will be lessthan 0.1 vol. % (measurement in the blank by SEM).

Adding O₂ to the dehydration/consolidation gases helps to retain TiO₂ inthe glass blank and also induces growth of anatase. It is believed thatO₂ does not prevent TiO₂ from migrating, but rather inhibits itsmigration and concomitant loss from the blank. O₂ is very important inachieving a relatively flat TiO₂ concentration profile for designs inwhich varying laydown concentration is not used to compensate fordepletion (see below). In addition, by using O₂ duringdehydration/consolidation, an alumina muffle may be employed. Theaddition of O₂ to dehydration/consolidation also increases the number ofanatase crystals from 2 to 4 times without correspondingly increasingthe vol. % anatase (the average anatase crystal in blanksdehydrated/consolidated with O₂ appears to be smaller than the averageanatase crystal in blanks dehydrated/consolidated without O₂).

The optimum lower level of O₂ concentration in the consolidation gasescorresponds to the amount of O₂ required for substantial inhibition ofdepletion, and this function may require only a very small concentrationof O₂. The optimum upper limit for O₂ concentration corresponds to an O₂level at which depletion inhibition is maximized for practical purposes.The rate of increase in the depletion rate appears to be inverselyproportional to the O₂ concentration. Tests have shown that the fatigueperformance of fibers made with dehydration/consolidation oxygen levelsfrom about 0.4 vol. % to about 20 vol. % exhibit identical fatigueperformance, while fibers made with dehydration/consolidation oxygenlevels below about 100 ppm show a marked degradation in fatigueperformance. Experimentation has shown the O₂ /Cl₂ ratios of as low as1:30 are sufficient to substantially reduce depletion of TiO₂. In orderto maintain good fatigue performance, the concentration of oxygen indehydration/consolidation is preferably at least about 0.03-0.3 vol. %for chlorine concentrations of about 1-10 vol. %

It should be noted that in some optical fiber designs, the use of highconcentrations of O₂ during dehydration/consolidation must be limiteddue to a deleterious effect on the optical performance of the resultingfiber (e.g., hydrogen effect attenuation increase). On the other hand,fibers drawn from blanks consolidated in O₂ appear to shown lessmigration of TiO₂ toward the center of the fiber and therefore, lessattenuation due to TiO₂. No more than about 5 vol. % O₂ concentrationshould be used to avoid the effects of hydrogen effect attenuation onthe optical performance of the fiber. Experiments have shown thatdehydration/consolidation oxygen concentrations in the range of about0.1 vol. % to 2.0 vol. % oxygen produce fibers which have acceptablehydrogen effect attenuation levels and appropriate fatigue resistance.And, for consolidation without O₂, a non-alumina muffle must be used toprevent contamination of the surface of the blank which would causesevere defects. O₂ helps to prevent the transport of alumina, therebylimiting surface attack.

As described above, there is a surprising increase in fatigue resistanceat TiO₂ concentrations above about 10-11 wt. %. However, high TiO₂concentrations may present severe manufacturing problems in normal sizeglass blanks with TiO₂ -SiO₂ layers greater than about 0.5 mm. For drawdown ratios of about 400 to 1, this corresponds with an outer claddingthickness of about 1 μm. Consolidation of such high concentration thicklayers (and of other combinations of TiO₂ concentration and thickness,e.g., greater than about 13.5% for outer cladding thicknesses of about3.5 μm), results in surface crazing, spalling, cracking and/orseparation of the outer cladding from the remainder of the glass blank.

The fiber layers discussed herein are typically cylindrical; in otherwords, they are axially symmetric at any particular radius.

FIG. 8 depicts, for draw down ratios of about 400 to 1, the combinationsof TiO₂ concentration and layer thickness which were more readilymanufacturable (solid square), and those combinations which gave rise tomanufacturing problems (open square). In addition, some fiber cleavingequipment encounters difficulties in cleaving thick layers with highTiO₂ concentration.

Moreover, as some proportion of fiber surface flaws are typically on theorder of about 1 μm (especially as the fiber length increases), suchflaws may initially pierce the thin layers of high TiO₂ so that theassociated crack tips are in the SiO₂ cladding and many of the crackinhibiting mechanisms of the TiO₂ -SiO₂ outer cladding are substantiallylost. As described below, one method of avoiding this problem for fiberswith thin TiO₂ -SiO₂ layers is to use increased proof stress levels toeliminate all flaws that are on the order of the layer thickness orlarger.

In order to compensate for the transport and depletion associated withchlorine, and to allow the use of a high concentration of TiO₂ withoutcrazing, spalling or surface bubbles, it is one aspect of our inventionto create a pre-selected laydown distribution with TiO₂concentration--in at least the outermost cladding layer--that is greaterthan the concentration desired in the resulting fiber. In a preferredembodiment of the invention, the total thickness of the outer claddingof the fiber is about 3.5 μm. The outer cladding comprises a first TiO₂layer approximately 3.1 μm thick with a concentration (as laid down) of6-10 wt. % TiO₂. In the last 0.4 μm of the outer cladding, the TiO₂concentration is increased by an additional 5-7.5 wt. % so that thetotal laydown concentration of this outermost higher concentration layeris 11-17.5 wt. %. These dimensions correspond as follows with laydowndimensions: 3.1 μm outer cladding bulk layer--8.1 mm soot layer; 0.4 μmhigher concentration layer--1.0 mm soot layer. As an alternative oraddition to this step increase, a ramp or other controllable method ofincreasing TiO₂ concentration in the depletion region may be employed.

The actual concentrations of TiO₂ as measured in the fiber are roughlythe same as those extrapolated from the deposition flows, with only aslight depletion in the outermost 0.06-0.08 μm of the fiber (50 μm ofthe blank). (The fiber depletion thickness is extrapolated from actualmeasurements on the blank.) For the purposes of this application, thesethin depleted regions are not considered to be separate fiber layers; a"layer" is defined to include a thicker region. For example, an"outermost layer" includes a significant layer thickness on the order of0.1 μm or greater which incorporates this thin depleted region.

Graphs of layer thickness vs. TiO₂ concentration as measured by electronmicroprobe are depicted in FIGS. 9a and 9b for one consolidated glassblank. The total layer thickness in the glass blank was approximately1.4 mm, which corresponds with a layer thickness in the fiber of about3.5 μm. The TiO₂ concentration as extrapolated from the deposition flowswas 7.5 wt. % for a 3.1 μm first layer (end-on SEM measurement gave aTiO₂ concentration of 8.6 wt. %) and 13 wt. % for a 0.4 μm outermostlayer. In FIG. 9a, the blank surface is at the right side of the graph.FIG. 9b is based on a higher resolution microprobe measurement of theoutermost surface layer in the same blank, with the blank surface at theleft side of the graph. FIG. 9b shows a slight depletion at the surface.The blank was dehydrated/consolidated in Cl₂ without O₂.

The n value for this fiber was measured to be 30.3. This relatively lown value again indicates that the thicker primary layer plays a veryimportant role in fatigue resistance. This measurement indicates that,for the same TiO₂ concentration in the outermost cladding layer, thehigher the TiO₂ concentration in the primary layer, the higher theresulting fatigue resistance.

A graph of fiber diameter vs. TiO₂ concentration as measured by STEMtechniques on the fiber is depicted in FIG. 10 for a fiber drawn fromthe glass blank depicted in FIGS. 9a and 9b. This graph gives a goodqualitative picture of the two layer structure present in the drawnfiber. It should be noted that for very thin layers, such as the 0.4 μmoutermost cladding layer, the actual concentration of TiO₂ is difficultto measure accurately in the fiber itself by STEM analysis because ofthe experimental sophistication required and the lack of internalcalibration to TiO₂ -SiO₂ standards. As shown in FIG. 10, the TiO₂concentration measurements of a lower concentration layer surrounded bya higher concentration layer are higher than expected from the blankprofile as measured by the accurate electron microprobe. However, a sideview electron microprobe of the fiber surface will always underestimatethe TiO₂ concentration in the higher concentration outer layer, if it isless than about 1.5 μm thick, the depth of the electron microprobe spot.

The following measurements were highly reliable and their predictedconfidence interval is given in parentheses: 1) measurement of fiberlayer thickness by SEM calibrated by NBS standard (±0.1 μm); 2)measurement of layer thickness in the consolidated glass blank (for theblank and the resulting fiber, as a function of draw down ratio) byelectron microprobe (±1 μm); 3) measurement of TiO₂ concentration byelectron microprobe in the consolidated glass blank (±0.1 wt. %); 4)measurement of the minimum TiO₂ concentration in at least one layer inthe outer 1 μm of the fiber by side-on electron microprobe (±0.1%); 5)measurement of TiO₂ crystal size (>about 0.3 μm) in the consolidatedglass blank by SEM (±0.1 μm); and, 6) measurement of inhomogeneity sizein the fiber by STEM (±10 Angstroms).

Other multiple layer structures are also beneficial. For example, asimilar thin layer with very high TiO₂ concentration could be placed ata depth corresponding to the maximum flaw size established while prooftesting. This would greatly inhibit strength degradation below the proofstress. The placement of this higher concentration layer can bedetermined using the following fracture mechanics relation,

    σ.sub.p =K.sub.Ic /0.73(πa).sup.1/2               (6)

where σ_(p) is the proof stress and a is the corresponding crack depth.For a discussion of crack depth vs. strength in general, see Glaesemann,Jakus, and Ritter, "Strength Variability of Indented Soda-Lime Glass",Journal of the American Ceramic Society, Vol. 70, No. 6, Jun. 1987, pp.441-444.

Thin, higher concentration outermost layers provide numerous advantages.In a thin outermost layer, greater TiO₂ concentrations are possiblewithout process problems in dehydration/consolidation (light bulbs,surface non-uniformity) and in draw (spalling, crazing, surfacenon-uniformity, non-uniform TiO₂ concentration). The diffusion of TiO₂from the surface of the blank in dehydration/consolidation iscompensated for. Higher TiO₂ concentrations result in more anatasecrystals and fines to form and in a depletion layer that begins closerto the surface of the blank (i.e., a thinner depletion layer). This isbelieved to be due in part to the resistance to Ti mobility provided bythe higher density of TiO₂ -SiO₂ layers with higher TiO₂ concentration.Another advantage is that for flaws contained within the coating layer,thinner coating layers of TiO₂ at higher concentrations appear toproduce higher fatigue resistance levels than thick coating layers atlower concentrations. The combination of all these attributes results inhigher TiO₂ concentration on the surface which equates to higher fatigueresistance in the final fiber.

C. ADDITIONAL FEATURES

As would be expected, the intrinsic strength of our inventive opticalfiber design with a TiO₂ -SiO₂ outer cladding is reduced about 25-70kpsi from that of a pure SiO₂ fiber (down from the SiO₂ values which arein the range of about 600-700 kpsi) as shown in FIG. 11 (note: theconcentrations plotted by the connected open squares are for both layersof a two layer outer cladding). It appears that this reduction inintrinsic strength does not change significantly with increasing TiO₂concentration. The test conditions for the strength measurements plottedin FIG. 11 are set forth above.

Intrinsic strength is determined by the behavior of "small flaws", whichby definition are of a size that is partially if not wholly determinedby the intrinsic structure of the glass at the surface. One explanationof the slight reduction in the intrinsic strength of fiber with a TiO₂-SiO₂ outer cladding compared to pure SiO₂ -clad fiber is the predicteddisrupted structure of the glass due to the addition of TiO₂. This isalso supported by the lower Young's modulus measured on such fiber (seeGlaesemann et. al., "Effect of Strain and Surface Composition on Young'sModulus of Optical Fibers", OFC Conference, 1988 Technical DigestSeries, Vol. 1, TUG5, Jan. 1988). It is therefore believed that theflaws associated with the high strength region are uniformly distributedover the entire glass surface. That means that regardless of the gaugelength of a fiber, a uniform doping with TiO₂ yields a finite upperlimit to strength that is less than that of SiO₂.

For most optical fiber applications, however, it is not the intrinsicstrength that is of primary concern but the frequency of breaks belowthe intrinsic strength region (the extrinsic strength). The manufactureof fiber with a TiO₂ -SiO₂ outer cladding in the manner described aboveresults in a significant reduction of extrinsic flaws. This is believedto be due to the reduction of draw furnace particle inclusions observedin fibers with a TiO₂ -SiO₂ outer layer.

Intrinsic strength (small flaw) analysis is also useful in understandingthe fatigue resistance of our inventive fiber. A given length of fiberhas only a single origin of failure and therefore, under axial tension,one intrinsic flaw is larger than all the others in that length offiber. This is confirmed by the strength decrease observed withincreasing gauge length, as the probability of a larger flaw occurringincreases with gauge length. Accordingly, the fiber will have thefatigue resistance associated with crack growth in an inhomogeneousfiber cladding material only if the worst flaw on the fiber length beingtested encounters an inhomogeneity. Therefore, the preferred fiberdesign will provide inhomogeneities of sufficient size and distributionthat a randomly occuring worst flaw in a given fiber length will alwaysencounter an inhomogeneity.

No decrease in the intrinsic strength region has been observed for ourinventive fibers compared with fibers having nominally flat TiO₂ dopedprofiles, no depletion, and TiO₂ concentration below about 10 wt. %. Onthe other hand, our inventive fibers yield extremely high n values.Therefore, we believe that intrinsic flaws are encounteringinhomogeneities which reduce their growth, but the inhomogeneitiesthemselves do not appear to affect extrinsic strength (note: acompressive stress effect for large flaws/low strengths is discussedbelow). Inhomogeneities are not a site for weakening, i.e., they do notnecessarily provide the site for the worst case flaw. Theinhomogeneities should be uniformly spread over the fiber surface tohave a 100% chance of encountering the largest intrinsic flaw.

The distribution in size and location of the inhomogeneities is aprimary determinant of whether the various inhomogeneity-induced crackgrowth resistance mechanisms will be effective in enhancing fatigueresistance. There are many ways of examining this problem, but weconjecture that the minimum inhomogeneity size will be determined by thesmallest inhomogeneity that can alter the stress field about the cracktip and that the maximum inhomogeneity size for a given volumepercentage of inhomogeneities is that size at which the probability of aflaw propagating through the layer and encountering a inhomogeneitybecomes appreciably less than 1. Concerning small inhomogeneities,consider a 600 kpsi flaw which has a depth of approximately 160Angstroms. It is conjectured that the minimum inhomogeneity size neededto affect the stress field in this case is about 10 to 16 Angstroms.

To estimate the maximum inhomogeneity size it is important to note thatthe TiO₂ concentration and fiber processing conditions determine thevolume percentage of inhomogeneities. We now consider all flaws within a0.5-3.5 μm outer cladding layer encountering randomly (not uniformly)dispersed inhomogeneities. One method of arriving at the maximuminhomogeneity size is to calculate the probability that all flaws alongan one kilometer length of fiber will not encounter an inhomogeneitybefore traversing the layer, and hence, will not experience enhancedcrack inhibition. For one volume percentage of inhomogeneities we haveobserved in our inventive fiber (around 10%), to ensure that all flawshave a high probability of encountering an inhomogeneity, the averageinhomogeneity size should be less than about 100 Angstroms in diameter(for 20 vol. %, the average inhomogeneity size should be less than about250 Angstroms). Inhomogeneities of a size larger than this limit,although effective in hindering the propagation of a given flaw that isdeliberately placed near a inhomogeneity, do not optimally use theavailable TiO₂ to provide the highest confidence that all flaws anywherein the layer will be affected by a inhomogeneity.

The average diameter of the inhomogeneities in the outer cladding layerof our inventive fibers are within the range of approximately 10-100Angstroms. Preferably, a substantial portion of the inhomogeneities arewithin the range of from 30-50 Angstroms. The size of theinhomogeneities may be measured by STEM techniques to ±10 Angstroms.X-RAY diffraction techniques may be used to discern whether anysubstantial fraction (greater than about 0.1 vol. %) of inhomogeneitieswith diameters greater than about 200 Angstroms is present in the outercladding layer, as the resolution minimum for X-RAY diffractiontechniques is on the order of 200 Angstroms.

For reliability purposes, optical fibers are usually proof tested toestablish a maximum flaw depth. In the context of our present concern itis desirable to have the maximum flaw contained within the TiO₂ -SiO₂layer over the life of the fiber. In the case of no flaw growth duringthe in-service life, the layer depth is equal to the maximum crack depthwhich can be determined from the fracture mechanics relationship setforth above,

    σ.sub.p =K.sub.Ic /0.73(πa).sup.1/2               (6')

where σ_(p) is the minimum strength taken to be the proof stress, a isthe crack depth, and K_(Ic) is the fracture toughness which is taken tobe 0.7 MPa m^(1/2). Table I gives crack depth for a range of proofstresses.

                  TABLE I    ______________________________________    TiO.sub.2 --SiO.sub.2  Crack Depth as a Function of Proof Stress    Layer Depth    Proof Stress    (μm)        (kpsi)    ______________________________________    2.5             50    1.7             60    1.3             70    0.6            100    0.2            200     0.07          300    ______________________________________

Thus, from Table I it can be seen that all surface cracks would be lessthan 1.3 μm deep after proof test at 70 kpsi and therefore all surfacecracks would be completely contained in a 1.3 μm TiO₂ -SiO₂ outercladding layer. In addition layer depth can be extended to accommodateanticipated crack growth over the fiber life. For example, 10% crackgrowth over 40 years from a minimum strength of 50 kpsi would require alayer depth of 2.8 microns.

As discussed above, the conventional explanation for improved fatigueresistance in optical fibers with TiO₂ -SiO₂ outer layers has been thecompressive stress resulting from the mismatch in the thermalcoefficients of expansion for the SiO₂ and TiO₂ -SiO₂ layers. We havefound that this effect is not substantial in our inventive fibers forsmall flaws (corresponding with strengths and proof stresses greaterthan about 150 kpsi).

We have measured the compressive stress in representative samples of ourfiber as follows:

    ______________________________________    8/12 wt. %    3.1/0.4 μm                             12,840 psi                                       [n = 30]    (two layer)    14.7/16.7 wt. %                  3.1/0.4 μm                             16,080 psi                                       [n = 87]    (two layer-FIG. 2a)    17.4 wt. %    1.1 μm   5,880 psi                                       [n = 80.3]    (single layer-FIG. 2b)    ______________________________________

For large flaws below the 150 kpsi minimum proof stress level, it isbelieved that compressive stresses in the range of 15-20 kpsi may play arole in fiber strength (break rate) and fatigue resistance. In practicalterms, a fiber subjected to a low stress (less than 65-70 kpsi) duringcabling or subsequent use would see a benefit. For example, large flawsremaining after proof stress that would normally grow critically at65-70 kpsi bending or tensile stress would only see a stress of about 50kpsi.

This may provide an advantage for applied stresses near the proof stresslevel inasmuch as residual compressive stresses may enhance the fatigueresistance of a fiber with poor strength. For such fibers, the measuredn value may be higher than the n value determined from the materialcomposition by an amount corresponding with the compressive stress.Thus, the introduction of a residual compressive stress on the outsideof the glass cladding leads to better apparent fatigue behavior forlarge flaws than for small flaws. However, for handling and reliabilitypurposes it is still desirable to have as few large flaws as possible.

On the other hand, for higher bending/tensile stresses that are appliedto the fiber in high proof stress applications, the residual compressivestress provides little or no benefit. For the small flaws that areassociated with such higher stresses, the compressive stress is quicklyovercome and subcritical crack growth ensues. This is the case for fiberapplications which require proof stresses in the range of 150-300 kpsi,such as undersea cables, local area networks, and specialty applicationssuch as gyroscopes or wound fiber bobbins for tethered missiles.

In addition, we have found that our inventive fibers with higher TiO₂concentrations have substantially improved abrasion resistance.Therefore, these fibers will be less likely to develop large, lowstrength flaws due to improper handling.

Conventional techniques for calculating residual compressive stress,such as measurement of the thermal coefficient of expansion mismatch bytrident seal techniques, are not amenable to glass optical fiber.Therefore, we measured the state of residual stress (compression ortension) of the titania-silica layer directly using a photoelastictechnique which did not require knowledge of the coefficient of thermalexpansion. This technique requires that the layer be transparent andthat optical retardation differences be measurable within the layer.

Using a polarizing microscope, the axial stress within the layer can becalculated from the following equation: ##EQU2## where σ=stress in psi,

A=compensator angle in degrees

K=stress optical constant, 0.292 nm/cm/psi

ρ=optical path in cm.

The optical path length was calculated by

    ρ=2(Da).sup.1/2                                        (8)

where a is the layer thickness and D is the thickness of the inner(silica) body.

The stress optical constant, K, was calculated by extrapolating theknown values for ULE (code 7971) glass (8 wt. % TiO₂) and fused silica(code 7940). The value was limited to 0.292 nm/cm/psi to avoidunderestimating the stress. Thus, the calculated stress values areexpected to be, if anything, over-estimated.

The optical retardation or birefringence was determined by rotation of acompensator in the microscope where one degree of rotation is equal to3.15 nm of retardation. A stress can be computed with a precision of±10%. The determination of whether a stress is compressive or tensilewas determined with a calibrating glass bar.

The compressive stress for a fiber with a homogeneous glass layer ofTiO₂ -SiO₂ 2.5 μm thick with 8.7 wt. % TiO₂ was measured to be 8.63 kpsiby the above technique.

D. EXAMPLES

The following are examples of embodiments of our inventive design andmanufacturing process.

EXAMPLE 1

In one embodiment of the invention, fibers were made by the followingprocess. First, a large diameter (8.1 mm) intermediate fiber wasproduced by the process described in Berkey U.S. Pat. No. 4,486,212.This intermediate fiber, comprising the core and a portion of thecladding in the resulting fiber, was placed in an overcladding lathe forthe deposition of SiO₂ soot as further described in U.S. Pat. No.4,486,212. The overcladding lathe rotated the intermediate fiber infront of three pairs of soot deposition burners which traversed back andforth in front of the intermediate fiber on three shuttles spaced at 45°angles along a 90° arc. The two burners in each pair were fixed inrelation to each other. The optimum shuttle speed was 2.0 cm/sec., andthe intermediate fiber rotated at 150 rpm. The burners were similar tothose described in connection with FIG. 1 in Powers, Sandhage andStalker U.S. patent application Ser. No. 435,966, filed Nov. 13, 1989,which is copending with the present application (see also BlankenshipU.S. Pat. No. 4,314,837 and the patents referred to in U.S. Pat. No.4,486,212). In this manner, the intermediate fiber was overcoated withSiO₂ soot to a diameter of 108-118 mm.

Thereafter, two of the pairs of burners were turned off and a TiO₂ -SiO₂outer layer was laid down by the single shuttle in two stages ofreactant flows. In a first deposition stage a layer with TiO₂concentration centered at 8 wt. % was created to a layer thickness ofapproximately 8.1 mm (this corresponds with about 3.1 μm in the drawnfiber). In a higher concentration stage after the first stage, a secondlayer with TiO₂ concentration centered at 14.5 wt. % was laid down to athickness of approximately 1.0 mm (this corresponds with 0.4 μm in thedrawn fiber). The total thickness of the TiO₂ -SiO₂ layer was 9.1 mm.

The reactant delivery system was of the type described in BlankenshipU.S. Pat. No. 4,314,837. In addition, a flash vaporization system, asdescribed in the copending Antos et al U.S. patent application Ser. No.07/456,118, entitled Flash Vaporizer System for Use in ManufacturingOptical Waveguide Fiber, was incorporated to deliver the TiCl₄ vapor.FIG. 12 depicts the face 11 of one of the burners used in this process,with central fume tube 13, inner shield annulus 15, fuel pre-mixorifices 17 and outer shield orifices 19.

The optimum flows to each burner during deposition of the TiO₂ -SiO₂layer were as follows:

    ______________________________________    Fume tube SiCl.sub.4 :                       23.83  gm/min.    Fume tube O.sub.2 :                       2.83   std. liters/min.    (with SiCl.sub.4)    Fume tube TiCl.sub.4 :                       1.5    gm/min.    (first stage)    Fume tube TiCl.sub.4 :                       2.87   gm/min.    (higher conc. stage)    Fume tube O.sub.2 :                       1.0    std. liters/min.    (with TiCl.sub.4)    Inner shield O.sub.2 :                       2.9    std. liters/min.    Pre-mix O.sub.2 :  16.67  std. liters/min.    Pre-mix CH.sub.4 : 20.0   std. liters/min.    Outer shield O.sub.2 :                       6.6    std. liters/min.    ______________________________________

Between the deposition of the first layer and the second higherconcentration layer, the soot preform is allowed to cool for a period ofpreferably approximately 10 minutes, in order to allow the TiCl₄ flow tostabilize at the new set point. It is believed that this helps toproduce a uniform step interface between the two layers and to increasethe level of TiO₂ crystal capture at the interface.

The process sequence was carried out as follows. First, SiO₂ wasdeposited for 88.5% of the overcladding deposition weight. Second, theTiCl₄ flow was stabilized by flowing into a vent before merging with theSiCl₄ system. After stabilization, the TiCl₄ /O₂ mixture was merged intothe SiCl₄ line connected with the single shuttle that was depositing theTiO₂ -SiO₂ soot. Deposition of the first stage took place from 88.5 wt.% of the overcladding deposition to 98.5 wt. %. Thereafter, thedepositing shuttle returned to its starting position and the TiCl₄ flowwas restabilized to the higher concentration stage flow. Afterstabilization, the shuttle traversed the blank for three passes (onepass is one stroke up and one stroke down). The final diameter of thesoot preforms ranged from 108 to 118 mm.

The soot preform was next introduced into a dehydration/consolidationfurnace as described in Lane et al. U.S. Pat. No. 4,741,748, relevantportions of which are incorporated herein by reference. In this process,0.3 std. liters/min. Cl₂ and 40 std. liters/min. He were usedthroughout, and no O₂ was introduced into the furnace.

In a first oscillating coil mode, the coil traversed the entire blank ata temperature of approximately 1100° C. for about 20 minutes.Thereafter, the coil temperature was increased to approximately1400°-1450° C. and driven from the bottom of the blank up at a velocityof about 7 mm/min. This peak consolidation temperature was slightlylower than for standard SiO₂ clad blanks, because of the lower viscosityof the TiO₂ -SiO₂ and to allow complete dehydration prior to glaze overof the TiO₂ -SiO₂ layer at the blank tip. After the blank was completelyconsolidated, there was a 5 minute purge of any residual Cl₂ using Heand N₂ so that the blank could be unloaded safely. After the blank wasremoved from the consolidation furnace, it was kept for at leastapproximately 6 hours in a holding oven at 850° C. in an atmosphere ofair, prior to drawing. The holding oven step is preferable but it is notbelieved to be required.

The consolidated blank diameter ranged from 50 to 60 mm. The diameter ofthe first stage layer ranged from 1.2 to 1.5 mm, with the higherconcentration stage layer comprising the last 0.2 to 0.25 mm. The outer50 μm layer was depleted to about 8 wt. % TiO₂, and the next 200 μm intothe surface was approximately 14.25 wt. % TiO₂. TiO₂ concentrationmeasurements were made by SEM.

The blank was then drawn into a fiber in a draw furnace. The draw handlewas modified according to the design described in Bailey U.S. Pat. No.4,126,436, to eliminate diameter upsets resulting from unstable thermalconditions in the blank near the handle. The fiber was coated by acoater of the type described in Kar et al. U.S. Pat. No. 4,531,959, andcoating bubbles were suppressed by a technique of the type described inDeneka et al. U.S. Pat. No. 4,792,347.

In the drawn fiber, the total fiber diameter was 125 μm, the entire TiO₂layer was 3.5 μm, the higher concentration stage layer was approximately0.4 μm, and the depleted layer was about 0.06-0.075 μm. The measured nvalues for the fiber averaged over 40.

The fiber with a measured n value of 87 that is depicted in FIG. 2a anddesignated by the open squares in FIGS. 1 and 11 was made by thisprocess with increased TiO₂ flows relative to the SiO₂ flows. A verysmall amount of O₂ is believed to have been present during thedehydration/consolidation process due to a leaky valve in an O₂ deliveryline that was programmed to be closed during the process.

EXAMPLE 2

In another example, the same fiber was manufactured but the first stageof the TiO₂ -SiO₂ outer cladding layer was deposited using all sixburners. The burners were arranged in pairs substantially the same as inthe first example with respect to SiO₂ deposition, but all six burnersoperated for the first stage of the TiO₂ -SiO₂ deposition. Three shuttledeposition resulted in higher rates of soot laydown. The TiCl₄ flow wassplit into three lines after leaving the flash vaporizer, and theselines were merged into the three SiCl₄ delivery lines for the pairs ofburners on each of the three shuttles.

The equipment and timing sequences for the three shuttle process aresubstantially the same as for the single shuttle process. At about 83%of the target preform weight, TiCl₄ was turned on to vent to stabilizethe TiCl₄ flow. TiCl₄ flow to the preform began at about 88.8% of targetweight. In the higher concentration stage, the flows to two of the threeshuttles were shut off at about 98.5% of the target preform weight andthe remaining shuttle operated as described above with respect to TiO₂-SiO₂ deposition in the first example.

The optimum flows to each burner during the deposition of the TiO₂ -SiO₂layer were as follows:

    ______________________________________    Fume tube SiCl.sub.4 :                       35     gm/min.    (first stage)    Fume tube SiCl.sub.4 :                       23.83  gm/min.    (higher conc. stage)    Fume tube O.sub.2 :                       1.5    std. liters/min.    (with SiCl.sub.4)    Fume tube TiCl.sub.4 :                       3.08   gm/min.    (first stage)    Fume tube TiCl.sub.4 :                       3.0    gm/min.    (higher conc. stage)    Fume tube O.sub.2 :                       0.67   std. liters/min.    (with TiCl.sub.4)    Inner shield O.sub.2 :                       2.9    std. liters/min.    Pre-mix O.sub.2 :  11.2   std. liters/min.    Pre-mix CH.sub.4 : 13.3   std. liters/min.    Outer shield O.sub.2 :                       6.6    std. liters/min.    ______________________________________

The optimum shuttle speeds were all 3.72 cm/sec. and the intermediatefiber rotated at about 275 rpm. An attempt was made to eliminate shuttleovertakes where one shuttle passed another while moving in the samevertical direction. The resulting soot preform had increased density inthe cladding layer and a lower density in the TiO₂ -SiO₂ layer than inthe first example. The layer thicknesses in the fiber were the same asin the first example. The n values for fibers made by this process werenot measured.

EXAMPLE 3

The thin layer process is substantially identical to

the single shuttle process with a few exceptions. The TiCl₄ flow isstabilized to vent at 90% of the target weight and TiO₂ depositionbegins at 96.8% of the target weight. The burner flows are the same asin the single shuttle process, except that the TiCl₄ flow is 4.6 gm/min.for each burner. The TiO₂ concentration was relatively uniform over asingle thin layer. The fiber layer thickness as measured by SEM wasabout 1.0-1.2 μm, and the TiO₂ concentration as extrapolated from thedeposition flows was about 17.4 wt. %, although a side-on SEMmeasurement of one of the fibers indicated a TiO₂ concentration of 15.8wt. %. Two of the fibers made by this process had measured n values of76.8 and 80.3 and are designated in FIGS. 1 and 11 by open diamonds (seealso FIG. 2b).

EXAMPLE 4

In another embodiment of the invention, soot preforms were made on alathe that traversed the large diameter intermediate fiber back andforth in front of two stationary burners. The blank was traversed infront of the burners at a slow velocity in one direction (29 mm/min.)and then returned to the start position at a second fast velocity (1282mm/min.), so that deposition essentially took place only in onedirection. The spindle rotated at about 168 rpm.

During deposition of the SiO₂ cladding layer, at about 81.5% of thetarget blank weight, the TiCl₄ flow was stabilized to vent, and TiO₂deposition began at 88.7% of target weight. The average soot preformdiameter was about 80.6 mm.

The optimum flows to each burner during the deposition of the TiO₂ -SiO₂layer were as follows:

    ______________________________________    Fume tube SiCl.sub.4 :                      23.7   gm/min.    Fume tube O.sub.2 :                      2.8    std. liters/min.    (with SiCl.sub.4)    Fume tube TiCl.sub.4 :                      1.45   gm/min.    Fume tube O.sub.2 :                      1.25   std. liters/min.    (with TiCl.sub.4)    Inner shield O.sub.2 :                      2.5    std. liters/min.    Pre-mix O.sub.2 : 9.9    std. liters/min.    Pre-mix CH.sub.4 :                      12.05  std. liters/min.    Outer shield O.sub.2 :                      5.0    std. liters/min.    ______________________________________

In this example, the soot preform was dehydrated/consolidated in astationary hot zone furnace of the type described in DeLuca U.S. Pat.No. 3,933,454 and Powers U.S. Pat. No. 4,125,388.

The preform was quickly lowered into the top of the furnace and heldthere for 8 minutes while the gas flow is stabilized at:

    ______________________________________    O.sub.2           1.17 std. liters/min.    He               41.23 std. liters/min.    Cl.sub.2          0.34 std. liters/min.    ______________________________________

The peak hot zone temperature was about 1590° C. The minimum temperatureat the top of the furnace was about 800° C. The preform was then drivendown into the hot zone at a downfeed rate of approximately 7 mm/min. Allof the dehydration/consolidation gases continued to flow. The Cl₂ flowwas shut off after about 190 minutes, and the blank was held in a bottomhold position for 15 min. (the top of the blank was in the hot zone).After the 15 min. hold time elapsed, the blank consolidation wascomplete and the blank was pulled up out of the furnace.

Fibers made by this process had approximately 3.5 μm TiO₂ -SiO₂ outercladding layers with TiO₂ concentrations measured to be 12.6 and 13.4wt. % by extrapolation of the deposition flows. The average n value was45.6. Two of the fibers made by this process are designated by the opencircles in FIG. 1 (see also FIG. 2c).

There have been various physical dimension changes to the bump inalternative designs. We have made blanks with two higher concentrationlayers (with same overall thickness as standard) and higherconcentration layers which are a constant ramp of TiCl₄ flow/TiO₂concentration. We have also made fiber with higher concentrationoutermost cladding layers from 0.18 to 0.8 μm thick and having varyingTiO₂ concentrations.

The present invention has been particularly shown and described withreference to preferred embodiments thereof, however, it will beunderstood by those skilled in the art that various changes in the formand details may be made therein without departing from the true spiritand scope of the invention as defined by the following claims.

We claim:
 1. A method for manufacturing a fatigue resistant opticalwaveguide fiber with a TiO₂ -SiO₂ outer cladding, comprising,forming adoped SiO₂ preform with a core portion and a cladding portion,depositing a layer of TiO₂ -SiO₂ soot with a TiO₂ concentration profileon the outside of said cladding portion to create an augmented preform,said TiO₂ -SiO₂ soot layer including at least one cylindrical axiallysymmetric layer with thickness of about 0.1 μm or greater having a TiO₂concentration greater than 10.5 wt. %, exposing said augmented preformto an atmosphere containing chlorine and oxygen at a temperature withinthe range of about 900° to 1400° C., said oxygen being present in aconcentration that is sufficient to substantially maintain said TiO₂concentration profile of said TiO₂ -SiO₂ layer, consolidating saidpreform into a glass blank with a TiO₂ -SiO₂ outer cladding layer, anddrawing said glass blank with a TiO₂ -SiO₂ outer cladding layer into anoptical waveguide fiber with inhomogeneities in the outer TiO₂ -SiO₂layer of the fiber.
 2. The method of claim 1 wherein said exposing stepfurther comprises exposing said augmented preform to an oxygencontaining atmosphere comprising at least about 0.03 vol. % oxygen. 3.The method of claim 2 wherein said exposing step further comprisesexposing said augmented preform to an oxygen containing atmospherecomprising at least about 0.1 vol. % oxygen.
 4. The method of claim 1wherein said exposing and consolidating steps are carried out in afurnace with an alumina muffle.
 5. The method of claim 1 wherein saidexposing step further comprises exposing said augmented preform to anoxygen containing atmosphere whose oxygen concentration is below a levelwhich results in substantially no increase in hydrogen effectattenuation in said optical waveguide fiber.
 6. The method of claim 5wherein said exposing step further comprises exposing said augmentedpreform to an oxygen containing atmosphere comprising from about 0.03vol. % to about 2.0 vol. % oxygen.
 7. The method of claim 6 wherein saidexposing step further comprises exposing said augmented preform to anoxygen containing atmosphere comprising from about 0.1 vol. % to about0.3 vol. % oxygen.
 8. A method for manufacturing a fatigue resistantoptical waveguide fiber with a TiO₂ -SiO₂ outer cladding,comprising,forming a doped SiO₂ preform with a core portion and acladding portion, depositing a layer of TiO₂ -SiO₂ soot with a TiO₂concentration profile on the outside of said cladding portion to createan augmented preform, said TiO₂ -SiO₂ soot layer including at least onecylindrical axially symmetric layer with thickness of about 0.1 μm orgreater having a TiO₂ concentration greater than 10.5 wt. %, exposingsaid augmented preform to an atmosphere containing chlorine and oxygenat a temperature within the range of about 900° to 1400° C., said oxygenbeing present in a concentration sufficient to substantially reduce thedepletion of TiO₂ from said augmented preform that results from thepresence of chlorine in said exposing step and/or to substantiallyreduce the migration of TiO₂ toward the center of said augmented preformthereby substantially maintaining said TiO₂ concentration profile ofsaid TiO₂ -SiO₂ layer, consolidating said preform into a glass blankwith a TiO₂ -SiO₂ outer cladding layer, and drawing said glass blankwith a TiO₂ -SiO₂ outer cladding layer into an optical waveguide fiberwith inhomogeneities in the outer TiO₂ -SiO₂ layer of the fiber.
 9. Themethod of claim 8 wherein said exposing step further comprises exposingsaid augmented preform to an atmosphere containing oxygen in aconcentration of at least about 0.03 vol. % oxygen.
 10. The method ofclaim 9 wherein said exposing step further comprises exposing saidaugmented preform to an atmosphere containing oxygen in a concentrationof at least about 0.1 vol. % oxygen.
 11. The method of claim 8 whereinsaid exposing step further comprises exposing said augmented preform toan oxygen containing atmosphere whose oxygen concentration is below alevel which results in substantially no increase in hydrogen effectattenuation in said optical waveguide fiber.
 12. The method of claim 11wherein said exposing step further comprises exposing said augmentedpreform to an oxygen containing atmosphere comprising from about 0.03vol. % to about 2.0% oxygen.
 13. The method of claim 11 wherein saidexposing step further comprises exposing said augmented preform to anoxygen containing atmosphere comprising from about 0.1 vol. % to about0.3 vol. % oxygen.
 14. The method of claim 8 wherein said exposing andconsolidating steps are carried out in a furnace with an alumina muffle.